The present invention relates to a magnetic recording medium and a magnetic recorder. The invention more particularly relates to a magnetic recording medium and a magnetic recorder, which are suitable for information recording with a thermally assisted magnetic recording technique.
To make the capacity of a magnetic recorder such as a hard disk drive (HDD) higher and reduce the cost of the magnetic recorder, recording density of a recording medium needs to be improved. The HDD includes a recording medium having a thin magnetic film that serves as a recording layer. The HDD uses, as bit information, a magnetization state of each magnetic particle (magnetic cluster including at least one crystal grain) included in the magnetic film. In order to increase the recording density in HDDs, it is necessary to reduce the size of each of the magnetic particles. When each magnetic particle is made too small, however, the thermal stability of magnetic particles deteriorates significantly; therefore a recorded magnetization direction may become inaccurate, and recorded information may disappear. For this reason, reduction in the size of each of the magnetic particles is limited. It is believed that the size of the magnetic particles used in the existent techniques is close to the limit of size reduction.
When magnetic anisotropy energy is represented as KU and the volume of the magnetic particle is represented as V, magnetic energy of the magnetic particle is equal to KU multiplied by V. When the magnetic energy of the magnetic particle is significantly larger than thermal energy (KB multiplied by V) (KB is Boltzmann constant, and T is temperature), the thermal stability of the magnetic particle during magnetization is maintained. Hence, it can be expected that a magnetic material having magnetic anisotropy energy KU higher than a magnetic material used in a conventional technique is used for a recording layer, thereby reducing the size of each magnetic particle. In this case, the magnetic anisotropy energy is proportional to a coercive force of the magnetic particle. Thus, the intensity of a magnetic field (recording magnetic field) generated for recording by a magnetic head needs to be increased to a level higher than in the conventional technique. It is, however, believed that increase in the intensity of the recording magnetic field is approaching the limits in the state of the art, irrespective of an attempt of enhancing the intensity of the magnetic field.
To solve the problem described above, thermally assisted magnetic recording has been studied in recent years. The thermally assisted magnetic recording technique uses a phenomenon that the coercive force of a magnetic material decreases when the temperature of the magnetic material is high. To perform the thermally assisted magnetic recording, a magnetic material having high magnetic anisotropy energy is used in a magnetic recording medium. Magnetic particles contained in the magnetic material are heated by means of laser light or the like during recording. Thus, the magnetic recording can be performed even with a conventional magnetic field intensity level. The thermal stability of the magnetic particles is maintained by cooling the magnetic particles, as the magnetic material has a high coercive force. However, there are many challenges to be addressed for putting the thermally assisted magnetic recording into practical use (refer to “Review of Thermally Assisted Magnetic Recording” written by Keiji Shono and Mitsumasa Oshiki, Journal of the Magnetics Society of Japan, 2005, Vol. 29, No. 1, pp. 5-13).
An important challenge to be addressed for putting the thermally assisted magnetic recording into practical use is to effectively heat and cool magnetic particles. As long as the challenge is not solved, the temperature of the magnetic particles after heating and recording remains high and the thermal stability of the magnetic particles decreases. In addition, heat is transferred to magnetic particles located near the magnetic particles heated for recording and the thermal stability of the magnetic particles located near the magnetic particles decreases. This results in recorded information disappearing.
It is, therefore, an object of the present invention to provide a high recording density and high reliable magnetic recording medium (for HDD) into which data is recorded with a thermally assisted magnetic recording technique, and to provide a magnetic recorder having the magnetic recording medium. The magnetic recording medium has a recording layer containing magnetic particles. The magnetic particles are effectively heated and cooled for magnetic recording. The magnetic recording medium ensures thermal stability of magnetic particles heated for magnetic recording and thermal stability of magnetic particles located near the magnetic particles heated for magnetic recording so as to suppress disappearance of magnetically recorded information.